The main stream of photo voltaics produced at present is silicon crystal type. In the production process of the crystal type photo voltaics, there are various steps of an ingot-making step wherein a high-purity silicon material is temporarily melted and then resolidified, a block-making or wafer-making step wherein the resulting ingot is subjected to cutting or slicing, a cell-making step wherein the resulting wafer is imparted with a function of battery, and a module-making step wherein the resulting cells are arranged to give a structure capable of being practically set up.
In the ingot-making step, two kinds of processes of a single crystallization process using CZ method and a multicrystallization process using casting method or Bridgman's method are known as typical processes. In any crystal type of single-crystal type and multicrystal type, the steps of a process for producing a silicon ingot are broadly classified into a step of packing a high-purity silicon material into a crucible every production batch to obtain one ingot, a step of supplying heat energy from the outer periphery of the crucible to melt the packing, a step of re-solidifying the melt with paying attention to crystal growth and a step of solidifying the resolidified silicon ingot and taking it out of the crucible.
In the ingot-making step, properties of the high-purity silicon material have great influence particularly on the time required for the step of melting the material among the above steps, and this greatly influences the ingot production efficiency itself.
As the high-purity silicon material, granular silicon having a particle diameter of about 1 mm that is produced by fluidized bed method, bar silicon obtained by breaking a silicon rod that is produced by Siemens method, crushed silicon obtained by crushing the bar silicon into fragments of about 3 to 50 mm, scrap silicon mainly constituted of residues and scraps obtained in an ingot-making step, or the like is used at present.
When the granular or crushed silicon is used as the high-purity silicon material, an extremely large number of particles must be packed because the particles are small. However, a joint of particles therebetween has low thermal conductivity, and as the number of the particles packed is increased, the number of joints of particles becomes larger. Consequently, a layer of the packed particles inevitably has bad thermal conductivity, and heating of the whole packing layer to a temperature in the vicinity of the melting point requires an extremely long period of time.
Further, because the small particles themselves have a large surface area, they have properties that once the surrounds of the particles are exposed to a melt, the particles are melted relatively easily. In the packing state where joints of particles are present, however, there is a problem that if the temperature is raised to a certain extent in the heating process, the joints are sintered together to keep the shape of the packing, and the bad thermal conductivity is also continued. That is to say, even if melting of silicon begins at the inner periphery adjacent to a crucible heated, a packing layer of the joined small particles having a low temperature is still present inside the crucible, and a considerable time is required before every particle inside the packing layer is wetted by the melt.
For the above reason, in the use of a silicon packing layer of small particles, there are problems that a long period of time is required from the beginning of melting to the completion of melting and the production efficiency is low.
On the other hand, the bar silicon or the silicon mass itself has good thermal conductivity and is relatively rapidly heated to a temperature in the vicinity of the melting point, and melting of the silicon begins at the outer periphery of the silicon. Silicon, however, has a large quantity of latent heat of fusion (melting), so that even after the silicon mass is exposed to a melt having excellent thermal conductivity, the mass is gradually melted from its surface, and melting of the whole mass still requires a long period of time because the surface area of the mass is small for its volume.
In order to provide multicrystal silicon which does not do damage due to floating or falling of a silicon packing to a crucible when it is melted, Japanese Patent Laid-Open Publication No. 104711/2003 (patent document 1) discloses multicrystal silicon having a shape of a crucible, which is obtained by heat-melting a block of multicrystal silicon, crushed fragments thereof or a mixture of the block and the fragments in a crucible and then solidifying the silicon in the crucible to solidify and mold it in a shape of a crucible. In this process, however, a block of multicrystal silicon or the like is used, so that the problems that the apparent density is too high and much time is required to melt the silicon have not been solved.
In Japanese Paten Laid-Open Publication No. 314996/1999 (patent document 2), there is disclosed a process for producing silicon single crystals and multicrystals using a gas phase component material, more specifically, a process for producing crystals in which a heat-generating solid, a high-frequency coil arranged opposite to a lower surface of the heat-generating solid and at least one gas-supply opening arranged on the coil surface are provided and which comprises induction-heating the heat-generating solid by the high-frequency coil to a temperature of not lower than the melting point of a deposition component element or compound, blowing a raw material gas containing at least one kind of the component element onto the lower surface of the heat-generating solid through at least one of the gas-supply opening to perform deposition or melting of the component element or compound on the lower surface of the heat-generating solid, and dropping the deposited melt or allowing it to flow downwards from the lower part of the heat-generating solid to produce crystals.
In the patent document 2, it is also disclosed that the melt thus dropped is received by a crucible, and with feeding the melt to the crucible, a multicrystal ingot is produced. In the patent document 2, it is further disclosed that by the use of a seed crystal or a single-crystal ingot, crystal growth is performed by a pulling method from the melt in the crucible to produce a multicrystal or single-crystal ingot. However, if the resulting ingot is taken out, the ingot exhibits a too high apparent density, and it takes an extremely long time to completely melt the ingot as previously described. Moreover, it is also suggested to feed the melt to the vessel drop by drop. However, as can be seen from the description that the vessel is heated and held, the apparent density of the resulting multicrystal silicon is high and a long period of time is sometimes required for melting.
In Japanese Patent Laid-Open Publication No. 316813/2002 (patent document 3), the present applicant has proposed a multicrystal silicon foam containing bubbles inside and having an apparent density of not more than 2.20 g/cm3. The silicon foam prepared by the process of this publication has an apparent density of the same level as that of the mass of the present invention, but it differs in that individual particles are smaller than those of the present invention. Depending upon the preparation conditions, the foams are fusion-bonded to one another to form a mass in certain cases, but this mass is very brittle and its thermal conductivity is not always high. When the foam is applied to an ingot-making step, the melting rate is improved than before. However, development of a silicon mass capable of further enhancing the ingot production efficiency has been desired.
In WO02/100777 (patent document 4), the present applicant has disclosed a process for producing silicon, comprising a step wherein a surface of a substrate is heated to a temperature lower than the melting point of silicon and the substrate surface is brought into contact with silanes with maintaining the temperature to deposit silicon and a step wherein the temperature of the substrate surface is raised to melt a part or all of the deposited silicon and thereby drop the silicon from the substrate surface and the dropped silicon is recovered. More specifically, there are disclosed a process (process 1) wherein silicon deposited on an inner wall surface of a cylindrical heated member is partially melted at the interface between the silicon and the reactor and dropped, and a process (process 2) wherein the whole amount of silicon deposited on a surface of a bar or V-shaped heated member is melted and dropped.
In the process 1, however, because the deposited silicon is melted at the interface between the silicon and the reactor, most of the silicon is dropped in an unmolten state, and therefore, the apparent density tends to become high. Actually, in Examples 1 to 5, a part of a deposit was melted and dropped, and in these examples, the apparent density of the resulting silicon exceeded 2.3 g/cm2.
The process 2 is a process wherein the whole amount of the deposited silicon is melted and dropped, and in Examples 6 and 7 shown as specific examples of the process, a melt of silicon fell dropwise owing to the shape of the heated member or the melting conditions, and the resulting silicon was close to the aforesaid silicon foam, so that there is room for improvement in strength and thermal conductivity.
As described above, the multicrystal silicon heretofore proposed has a disadvantage that when melting of the silicon is intended to produce an ingot, the silicon is hardly melted, and a long time is required for melting. On this account, there are problems of bad production efficiency and high energy cost. Further, there is another problem that if the temperature is raised to promote melting, a crucible itself is damaged to thereby inhibit normal ingot-making operations.
Patent document 1: Japanese Paten Patent Laid-Open Publication No. 104711/2003
Patent document 2: Japanese Paten Patent Laid-Open Publication No. 314996/1999
Patent document 3: Japanese Paten Patent Laid-Open Publication No. 316813/2002
Patent document 4: WO02/100777